The Finnish masonry heater tradition is several hundred years old. The down draft contraflow masonry heater currently used is well over one hundred years old. In the Finnish masonry heater design super heated gases move up a central fire tube or heater core to the top of the heater. Flue gases then drop down heat exchange channels on both sides of the heater core giving off heat to the exterior brick shell or wall of the masonry heater which in turn heats the surrounding space by convection, radiation and conduction. The cooled fluegases enter a common channel or manifold at the base of the heater and exit into a chimney flue behind or to either side of the heater. Once a burn is completed, dampers in the chimney flue are shut and the entire mass radiates heat for the next twelve to twenty-four hours.
A traditional Finnish masonry heater and bakeoven combination is illustrated in FIGS. 1 and 2. The masonry heater 10 is constructed with an outer shell or outer wall 12 of courses of common brick and mortar defining the outer surface or outer perimeter 14 of the masonry heater. The word "common" is used herein to refer to common construction bricks and portland cement based mortar not necessarily incorporating refractory materials. A double layer outer wall is shown at the sides of the masonry heater to conform with fire code requirements. Facades other than common brick may be used for the outer wall such as stone, soapstone, cast blocks, sheet metal, etc. The masonry heater 10 is also constructed with an inner core 15 of refractory material which confines the primary and secondary combustion flue gases. The refractory core 15 defines an inner perimeter 16 of the masonry heater. The inner core 15 is typically constructed of courses of refractory material firebrick mortared with a clay based refractory mortar.
The refractory core 15 is constructed to define a primary combustion firebox 18 with firebox access door 20 for combustion of, for example, wood fuel. An ash pit 22 of common brick and an ash clean out door 24 are provided below the firebox 18. The core construction also includes a bakeoven 25 above the firebox 18 with bakeoven access door 26 according to the masonry heater construction developed by Heikki Hyytiainen and Erkki Salmela of Finland. In this example the bakeoven door 26 is on the opposite side from the firebox door 20. The bakeoven 25 also functions as a secondary combustion chamber. If the chamber 25 functions only as a secondary combustion chamber and not as a bakeoven, then the bakeoven door 26 is eliminated. In the case of a bakeoven 25, the bakeoven door 26 may alternatively be provided on the same side as the firebox door 20.
Bakeoven flue outlets 28 are provided at the top of the bakeoven 25 leading to the contraflow flue channels 30. The contraflow flue channels 30 extend from the top of the masonry heater back to a chimney outlet manifold 32 at the bottom of the masonry heater. The chimney is coupled to the masonry heater at the bottom of the masonry heater where the temperature is lower and there is less heat stress on the chimney masonry heater joint.
The top of the bakeoven 25 is formed with an arched ceiling provided by a cast refractory bakeoven arch 27. Flue gases pass from the bakeoven 25 through the bakeoven flue outlet 28 and through a tertiary chamber 29 coupled with the contraflow flue channels 30. Combustion of flue gases is substantially completed as the flue gases pass through the tertiary space 29 to the contraflow flue channels 30 for exchange of heat to the outer shell 12 of the masonry heater. Air in the space being heated rises up the sides of the outer perimeter 14 in the opposite direction from the contraflow flue gases in channels 30.
The refractory core 15 is also constructed to provide a tapered throat section 35 which provides a flue coupling between the firebox 18 and secondary combustion chamber bakeoven 25. The tapered throat is generally constructed with straight sidewalls at the inner perimeter 16 of the core 15 over the sides of the firebox 18, and asymmetrical sloping front wall 36 and back wall 38. The tapered throat 35 therefore provides a relatively large flue opening at the base of the throat forming a flue outlet over the firebox 18 and a relatively small flue opening at the top of the throat forming a flue inlet to the bakeoven 25. The asymmetry of the sloping front wall 38 of tapered throat 35 has traditionally been used in order to permit enlargement of the firebox at the back of the masonry heater. At the same time, the flue inlet to the bakeoven is retained at one end to maximize continuous bakeoven floor area. The flue outlet from firebox 18 is therefore skewed to the front of the firebox.
There are several disadvantages associated with this traditional throat construction for masonry heaters. First, the asymmetry of the sloping front wall 36 and back wall 38 directs and reflects heat to the front of the firebox concentrating excessive thermal stress on the lintel 40 which supports the weight of the masonry heater over the firebox door 20. The lintel 40 is typically a cast refractory reinforced beam or a metal beam. This skewing of heat stress to the front of the firebox also complicates the design and construction of "see-through" type fireplaces and fireboxes. Second, thermal stress is distributed unevenly along the front and back walls of the throat. The asymmetry is aggravated as the firebox or fireplace is enlarged to the rear. Finally, the asymmetrical location of the flue inlet to the bakeoven 25 concentrates thermal stress on the inner wall of the bakeoven or secondary combustion chamber. Uneven distribution of thermal stresses may cause premature breakdown of the mortared refractory materials repairable only at great expense by tearing down portions of the masonry heater.
Further background and details of construction of Finnish fireplaces and the combination masonry heater and bakeoven can be found in the paperback books Finnish Fireplace Construction Manual 1984, by Albert A. Barden, III, published 1984 by Maine Wood Heat Company, Inc., RFD 1, Box 640, Norridgewock, Maine USA 04957; and Finnish Fireplaces Heart of the Home by Albert Barden and Heikki Hyytiainen, published by Heikki Hyytianen and Building Book Ltd., Finland, c/o Maine Wood Heat Company, Inc., RFD1, Box 640, Norridgewock, Maine USA 04957.
To alleviate some of these problems associated with the combination masonry heater and bakeoven of FIGS. 1 and 2, the present inventor developed a new tapered throat configuration with two major differences from the throat construction illustrated in FIGS. 1 and 2. First, in the Barden throat configuration manufactured by Maine Wood Heat Company, Inc., under the TM "Albiecore" the sloping sides of the tapered throat are symmetrical so that the relatively narrow flue opening at the top of the throat is centered over the relatively wider flue opening at the base of the throat. Second, the tapered throat is rotated 90.degree. so that the sloping sides of the throat are positioned over the sides of the firebox rather than the front and back walls.
The new Barden throat configuration affords several advantages. First the heat stress formerly concentrated over the lintel 40 and front of the firebox is displaced and redistributed to the center of the firebox from front to back. The thermal stress is distributed more evenly over the surfaces of the throat although the area over the lintel remains the hottest part of the masonry heater. Second, the firebox or fireplace may be enlarged to the rear while maintaining symmetry to any depth. Furthermore, the Barden throat configuration favors "see-through" firebox and fireplace designs. Third, the flue gases passing through the flue opening at the top of the throat which forms the flue inlet to the bakeoven can form two vortices or eddies for greater distribution around the surfaces of the bakeoven rather than concentrating the flue gases and consequent heat stress at the inner wall of the bakeoven.
Even with the improvements of the Barden configuration tapered throat with symmetrical sloping sides, the masonry heater throat remains the area of greatest concentration of thermal stress. Over time this may lead to incremental breakdown of refractory material firebricks or other refractory material lining the throat. Because of the location of the combustion throat in the heart of the massive masonry heater surrounded by the refractory core and outer shell, repairs can generally be achieved only by tearing down portions of the masonry work.